1. Field of the Invention
The present invention relates to a steer angle adjusting apparatus for a vehicle designed to adjust the steer angle of each of the right and left steered wheels of the vehicle in accordance with the vehicle running condition.
2. Description of the Related Art
In the Ackermann steering system, as shown in FIG. 13, the center O of turning of a left wheel 10L and that of a right wheel 10R are allowed to coincide with each other. Therefore, this steering system has the advantage that, when the vehicle is running at a relatively low speed, it can turn smoothly without any side slip of the wheels (see Japanese Patent Laid-Open No. 79758/1980).
In the parallel steering system, as shown in FIG. 12, the steer angle .theta..sub.i of an inner wheel and the steer angle .theta..sub.o of an outer wheel are allowed to coincide with each other. Therefore, when the vehicle is running at a relatively high speed, the difference between the slip angle of the left wheel 10L and that of the right wheel 10R is minimized and the cornering force is increased, so that the vehicle can turn smoothly.
Further, there has heretofore been one type of steer angle adjusting apparatus in which when the vehicle makes a turn, a steer angle which is proportional to a particular side slip angle of the rear wheels is given to them and at the same time, this steer angle is added to the steer angle of the front wheels. This conventional apparatus enables an improvement in the steering stability during high-speed running of the vehicle (see Japanese patent publication No. 10728/1965).
The above-described conventional systems suffer, however, from the following problems. Since the steer angles of the right and left steered wheels are unconditionally determined when the angle of steering rotation of the steering wheel is determined, it is possible to improve the steering stability only in a specific condition, which means that an adverse effect may be produced in conditions other than the specific one.
For example, the Ackermann steering system involves the problem that, when the vehicle makes a turn that is accompanied by a relatively large centrifugal force, the slip angle of the left wheel 10L, serving as an inner wheel, may be larger than an optimal value, resulting in an increase in the rate of wear of the tire, generation of tire skid noise and lowering of the steering stability.
In the parallel steering system, a slip angle may be generated when the vehicle turns at a relatively low speed, causing the steering wheel operability to be deteriorated.
In the above-described conventional steer angle adjusting apparatus, even when the vehicle turns at a relatively high speed, if the braking or driving force (hereinafter referred to as the "braking/driving force") is changed, the cornering force is also changed, and this leads to a change in the posture or attitude of the vehicle, disadvantageously.